A store sells bouquets of fruits for $10 a bouquet. The daily cost to produce x bouquets of fruits is defined by the function f(x) = -x4 + 3x3 + 5x2 + 20x + 1. What is the least number of bouquets that must be sold for the store to make a profit?
a. 3
b.4
c.5
d.6